[time-nuts] Checking accuracy of Rubidium standards

[Neville Michie]

Hi,
I have a plan which involves the dividing down of the 10MHz of a GPSDO
and a rubidium (LPRO) to about 1MHz or 100kHZ and applying them to a
XOR or D latch to get a PWM signal that can be averaged for a strip  
chart recorder or
12 bit analogue data logger. The DC output gives a range of 5 volts  
for one
microsecond or 10 microseconds phase difference and folds back if  
this difference is exceeded.
The data from the datalogger is in a format that a spreadsheet can use.
With time and phase measurements I wonder how hard it is to get Allen  
variance.
I realise the PWM method requires a low pass filter and this will  
prevent short period
variances from being calculated.
cheers, Neville Michie

On 09/11/2008, at 8:43 AM, Alan Melia wrote:

> This is an interesting thread again.....it may be similar to ones  
> that have
> been discussed, but one or two furthur questions occur to me. I have a
> Montronics sytem that does comparisons by the multiply and mix  
> process, and
> I find (also common to more modern Kethly systems) that the  
> limitation is
> around a part in 10^10 where the noise on the phase output makes it  
> not
> really usable (without a lot of averaging) being around or in  
> excess of 90
> degreees even with a couple of very good OXCOs. How does the 10G  
> comparision
> avoid this problem with standard multipliers? I doubt you can go  
> all that
> way with low-noise multipliers and have any useful signal left, or  
> have I
> missed something. At present I use a phase meter (lock in amps can  
> be quite
> good) at the MHz range and datalog the phase drift for several  
> hours. I have
> determined that setting "on the nose" is not necessary (for my
> applications). It is more useful to know how far a source is "off".
> How does the mix down compare with the seemingly more popular "mix  
> down and
> timestamp" I understand from previous threads that this has more  
> potential
> but might it also be as good even using simpler circuits that the NIST
> system.
>
> Thanks for all your efforts inthe background John..... great reading
> material !
>
> Alan G3NYK
>
> ----- Original Message -----
> From: "Jeffrey Pawlan" <jpawlan at pawlan.com>
> To: <hamradio at oz.net>; "Discussion of precise time and frequency
> measurement" <time-nuts at febo.com>
> Sent: Saturday, November 08, 2008 6:40 PM
> Subject: Re: [time-nuts] Checking accuracy of Rubidium standards
>
>
>>
>>
>> On Sat, 8 Nov 2008, Randy wrote:
>>
>>> I was wondering if it is worthwhile or even feasible to compare  
>>> an LPRO
>>> Rubidium standard against a Z3801.  Since their frequencies are  
>>> probably
>>> going to be extremely close anyway it would seem some special
>>> method/equipment would be required for high precision.  Suggestions?
>>>
>>>
>>> Randy, W7HR
>>> Port Orchard, WA
>>
>> The best way would be to compare the highest possible frequencies  
>> you can
>> generate with these two sources. I use two 10GHz sources that are  
>> each
> phase
>> locked to an external 10MHz reference. Then the 10GHz outputs can be
> compared
>> using either of these easy methods:
>> 1) look at the DC/IF output of a microwave mixer where the LO and  
>> RF ports
> are
>> driven by the two 10GHz sources. Don't overdrive the RF input to a  
>> level
> that
>> can burn out your mixer.
>>
>> 2) use a good microwave frequency counter to read one of the 10GHz  
>> outputs
> while
>> driving the counter's 10MHz ext ref input with the 10MHz from the  
>> other
> 10MHz
>> source. This is very fast but will only give you accuracy readings  
>> that
> are a
>> function of the resolution of the counter plus the bounce of the last
> digit
>> owing to sampling and triggering.
>>
>> 3) if you have access to a lab with one or two microwave synthesized
> signal
>> generators, then you can apply the 10MHz sources to the ext ref  
>> inputs of
> each
>> of these signal generators and then proceed as in 1) or 2)
>> I have done comparison at 26GHz this way so I have a bit more  
>> resolution.
>>
>> 73,
>>
>> Jeffrey Pawlan  WA6KBL
>>
>>
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